What is an Intrusion Dectection System?

An intrusion detection system (IDS) generally detects unwanted manipulations of computer systems, mainly through the Internet. The manipulations may take the form of attacks by crackers.

An intrusion detection system is used to detect several types of malicious behaviors that can compromise the security and trust of a computer system. This includes network attacks against vulnerable services, data driven attacks on applications, host based attacks such as privilege escalation, unauthorized logins and access to sensitive files, and malware (viruses, trojan horses, and worms).

Once detected security staff will be informed.

http://en.wikipedia.org/wiki/Intrusion_detection_system